The present invention generally relates to a laser system and more particularly to an ultra-fast laser system using a pulse shaper.
Conventionally, lasers used for chemical analysis through spectroscopy or mass spectrometry have used a laser beam pulse where the pulse duration and wavelength are fixed and computers are employed for simple chemical analysis processes. The laser beam pulse shape and, in particular the phase of the frequencies within its bandwidth, was not considered an important parameter and was not modified; whatever fixed shape was set by the manufacturer for the laser was used in the tests. The general concept of typically laser selective ion formation from molecules in a molecular beam is disclosed in the following publication: Assion et al., “Control of Chemical Reactions by Feedback-Optimized Phase-Shaped Femtosecond Laser Pulses,” Science, Vol. 282, page 919 (Oct. 30, 1998). The pulse shaping process with a learning algorithm is disclosed in Judson et al., “Teaching Lasers to Control Molecules,” Physical Review Letters, Vol. 68, No. 10, page 1500 (Mar. 9, 1992). It is noteworthy, however, that the Assion article discloses use of an 80 femtosecond laser pulse and requires molecules to be isolated in a molecular beam, while the Judson article discloses use of a one nanosecond laser pulse and is purely conceptual as it does not include experimental results. Similarly, the findings by Assion et al. had great scientific interest, but the results were not sufficiently reproducible to be considered useful for analytical purposes.
There have been recent experimental attempts to purposely shape the phase of ultrashort pulses since shaped pulses have been shown to increase the yield of certain chemical reactions and multiphoton excitation, although the mechanism for the observed changes remains unknown in most cases. As usually practiced, the output waveform is determined by the Fourier transform (hereinafter “FT”) of a spatial pattern transferred by a mask or a modulator array onto the dispersed optical spectrum. The introduction of liquid crystal modulator arrays and acousto-optic (hereinafter “A/O”) modulators into FT pulse shapers led to computer programmable pulse shaping, with millisecond and microsecond reprogramming times, respectively, and widespread adoption of this technique. These shaped pulses require a very large data set and in many cases, complex learning calculations for determining the pulse shaping characteristics for a particular application. The optimal pulse for the particular application is not known in advance. Since the variation shape of the possible pulse shapes is huge, scanning the entire parameter space is impossible and as such the optimized pulse shape could not have been predicted by theory. For a pulse shaper with N pixels, one can generate (P*A)N shaped pulses, where P and A are the number of different phases and amplitudes a pixel can take. If it is assumed 100 pixels, each taking 10 different amplitude values and 100 different phase values, the number of different pulses is of order of magnitude 10300. This dataset is extremely large, therefore, while in principle, the field exists to achieve the desired photonic transformation or excitation, finding it is a great challenge. Some researchers have attempted to avoid such complexity by binning together every 8 pixels on the pulse shaper, thereby converting a 128 pixel shaper into one with 16 active pixel groups, but with the inherent loss of accuracy. Therefore, it would be desirable for an ultra-fast laser system to control ultrashort pulses with a smaller dataset, operable to generate very complex pulse shapes that are optimal for the particular application and are highly reproducible. The following U.S. patent publications have overcome these traditional concerns and have led to reproducible results: 2004/0233944 entitled “Laser System Using Ultra-Short Laser Pulses,” published on Nov. 25, 2004; 2004/0089804 entitled “Control System and Apparatus for Use with Laser Excitation or Ionization,” published on May 13, 2004; and 2003/0099264 entitled “Laser System Using Ultrashort Laser Pulses,” published on May 29, 2003; all of which are incorporated by reference herein.
U.S. Patent Publication No. 2004/0145735 entitled “Coherently Controlled Nonlinear Raman Spectroscopy and Microscopy” to Silberberg et al. teaches use of a unitary pulse carrying a pump, Stokes and probe photon. This patent is incorporated by reference herein.
Additionally, monitoring the environment for chemical and biological agents, including explosives, from terrorist threats or from industrial contamination has become a necessity for reasons of national security and the well being of humans. Conventional devices are only designed for use to detect a single known agent or are inaccurate. Accordingly, to avoid a costly false positive or false negative identification, it would be desirable to employ an ultra-fast laser to environmental monitoring in order to quickly and accurately identify and/or act upon select molecules.